Transformer Differential Protection

Overcurrent and earth fault protective equipment employing time grading and directional detection cannot provide correct discrimination on all power networks and in many cases clearing times for some faults would not be acceptable. Differential protection is an alternative overcurrent protective scheme, which is used to protect individual sections of networks or pieces of equipment, such as transformers, generators, e.t.c. Thus, where protection co-ordination is difficult using time delayed over current and earth fault protection, or where fast fault clearance is critical, then differential protection may be used. Kirchhoff’s first law, which states that the sum of the currents flowing to a node must be equal to the sum of the currents flowing out from it is the basic principle of the differential protection scheme. It detects the difference between the current entering a section and that leaving it. Under normal operating conditions, the current leaving the protected unit would be equal to that entering it at every instant. If the current flowing into the protected unit is the same as the current leaving, then the fault is not in the protected unit and the protective equipment or relay should not operate. If there is a difference in either the phase or magnitude between input and output, then the fault is in the protected unit and the protection should operate. This paper investigates how power transformers can be protected using the current-differential protection schemes.